Chapter 2. LOX STORAGE SYSTEM FOR EXISTING AIP SUBMARINES

INTRODUCTION

Modern conventional submarines are fitted with Air Independent Propulsion (AIP) systems. In fact, AIP system is a small power plant providing for submarine the long term underwater operation. Output electric power of modern AIP systems is changed from 130kW to 600kW (as rule it is 300kW). The source of energy in AIP systems is a chemical energy of fuel. This energy is released in result of reaction of fuel with oxidant. There are some different methods transforming fuel energy into electricity. As oxidant in AIP submarines the oxygen is used in all cases. In AIP submarines pure oxygen is stored as rule in a liquid phase (LOx) in special tanks. However, oxygen can be stored in content of liquid or solid chemical composition as well.

Oxygen is generally liquefied so that it can be more effectively transported and stored in large volumes. However, most applications use oxygen after conversion liquid oxygen (LOX) into gaseous form (GOX).

Liquid oxygen is stored, shipped, and handled in several types of containers, depending upon the quantity required by the user.

The types of containers in use include the Dewar, Cryogenic liquid cylinder (CLC), and cryogenic liquid storage tank (CLT).

Storage quantities vary from a few liters to many thousands of gallons. Since heat leak is always present, vaporization takes place continuously. Rates of vaporization vary, depending on the design of the container and the volume of stored product. Containers are designed and manufactured according to the applicable codes and specifications for the temperatures and pressures involved .

There was suggestion to place the LOX tanks in the external main ballast tanks of
submarine. The proposal was investigated in Applied Physics Laboratory of Johns Hopkins University (1980).

LOX tanks located outside submarine naturally are heated by external heat sources.
They are: br>
solar gain and warm air when submarine is in surface position or it is warm water at submerged operating sub.

As for LOX tanks located inside submarine, the main heat source is warm internal air. There are two main types of tanks for LOX storage:

tanks with open vent system;

tanks with closed vent system

One of the main challenge of tanks with open vent system is continuous leakage of product (oxygen) as result of heat supply. Under the external heat, the pressure of oxygen in tank is increased. When pressure will reach given limit, the pressure‐relief valve is opened and part of gaseous oxygen is bled in atmosphere. LOX tanks with open vent system are often incorrectly referred to as “Dewars”. It should be remembered that the “Dewars” are open, nonpressurized vessels for holding cryogenic liquids.

A typical LOX tanks arrangements permits losses of oxygen through boiling approximately one percent of the volume per day. It should be noted that large tanks are more efficient in efforts to reduce the heat absorption by LOX. However, to place large LOX tank on submarine is more difficult task than to place some small tanks.

In vessels with closed vent system heating of LOX leads to so called thermal stratification. The cryogenic liquid is displaced from vessel by gas. Together with liquid displacement the heat and mass transfer processes have place on phases interface.

In vessels with closed vent system the thermo‐stating is the best way to avoid leakage. Gaseous oxygen is continuously passed through liquefier where it is transformed into liquid that is returned into tank. There are three main configurations of tanks for LOX storage:

vertical tanks;

horizontal tanks;

multi tanks system.

All tanks have cylindrical shape as rule. There are many different combinations of L / D from 10 to 0.7.

All existing AIP power generation systems contain the storage and feeding subsystem of oxygen. Currently, oxygen applied in AIP systems is stored in liquid form only. LOX is stored on AIP submarines in special cryogenic tanks. LOX tank consist of inner outer vessels. The space between vessels is filled with thermo insulation material.

LOX can be stored in tank at subcritical (below critical point — T<154.8K; p<50.9bar) and supercritical (upper critical point — T>154.8K; p>50.9bar) conditions.

Application of supercritical conditions for LOX storage has both advantages and drawbacks. Supercritical conditions exclude so called slosh of liquid in tanks. It can be used in submarines where sub trim have an influence on LOX feeding. However, high pressure LOX requires using more complicated and heavier LOX tanks.
In existing AIP power generation systems subcritical condition are used basically.

Currently, the AIP system containing compact power plant, the LOX and fuel tanks (as rule) is mounted in the single plug-sections that is welded in pressure hull of boat during submarine assembling. Such AIP plug-section usually has length from 8 to 15m and cross dimensions corresponding to submarine dimensions in the cutting-in place. It should be confessed that technology of AIP plug-section has both advantages and drawbacks obviously. We shall discuss the features of this technology in the chapter.

The present chapter contains description and analysis of existing LOX storage systems, their advantages and drawbacks. Analytical calculations of LOX tanks dinensions used in existing AIP systems had been performed as well. As for calculation results we should be noted that the given results are approximate and can be use for LOX storage system estimation only.
Chapter has some reccomendations to evaluate submerged endurance of submarine operating under AIP system.

The price of full version is $40. If you are interested in purchasing of full version of the article-presentation then contact me through e-mail:bakst.alexander[at]gmail.comor through contact form below.